1. Field of the Invention
The present invention pertains to optical systems for processing electromagnetic radiation. More particularly, the present invention relates to apparatus for separating a broad band beam of visible and infrared radiation so as to distinguish relatively narrow band segments of the beam for presentation to individual detectors operable within the corresponding narrow band regions. Such an optical system finds particular application in broad band optical scanning systems for use in video imaging systems.
2. Description of the Prior Art
Multispectral scanners for use with mechanical video systems are known and used, for example, in constructing pictures of ground terrain from a moving airplane. The operation of such a scanner combines with the forward motion of the plane to provide a sequence of views of the terrain passing below. An optical device as part of the scanner sweeps repeatedly in a direction perpendicular to the flight of the plane to gather electromagnetic radiation from the terrain along a range of directions in sequence. Such a sweeping optical device may be a rotating scan mirror which reflects the light gathered from below onto an optical system which, in turn, presents the gathered light to one or more electromagnetic radiation detectors. Thus, by use of the detectors and appropriate electronics, an image of the terrain may be reconstructed in much the same way a television image is constructed using a raster composed of horizontal sweeps of information-laden signals, with each successive horizontal sweep displaced laterally from the previous horizontal sweep. In such a mechanical scanner, the flight of the plane provides the analagous vertical displacement while the rotation of the scan mirror is used to construct the horizontal sweeps. The rate of the scan mirror rotation is proportional to the altitude of the plane as well as the speed of the plane. In a typical operation, such a scan mirror may rotate at a rotational speed ranging from, say, 600 rpm to 6,000 rpm.
The optical systems of such multispectral scanners should provide proper filtering to achieve well-defined wavelength limits of the band or bands of electromagnetic radiation desired to be detected. In practice, such wavelengths may range throughout the visible portion of the electromagnetic spectrum as well as the infrared spectrum. Due to the varied transmission properties of optical devices in these wavelength regions, and the general dependency of detectors on the wavelength of the radiation being detected, the radiation may be separated within the optical system of the multispectral scanner and various wavelength portions of the radiation presented to appropriate detectors. Consequently, known multispectral scanners have generally included large, relatively inefficient optical systems, or have been quite limited in the spectral ranges which they could effectively detect. Where relatively long focal length optics are employed, large image sizes in the optical systems of the scanners require detectors with relatively large sensing areas. Generally, background noise hampering the operation of such detectors is larger where such sensing areas are increased.
To reduce chromatic aberration as well as to minimize absorption losses, reflective gratings have been utilized to provide the necessary distinction among wavelength regions of the bands being detected. However, as is well known, the efficiency of such gratings varies with the wavelength distinguished. Consequently, a relatively flat frequency response is not available over a wide wavelength band using such a grating.
U.S. Pat. No. 2,895,049 discloses a camera operable in the infrared region of the spectrum.
U.S. Pat. No. 3,674,334 discloses a three-mirror optical system designed to provide high magnification in a relatively compact system. U.S. Pat. No. 3,927,254 discloses an optical system and scanning device including rotating dihedral and trihedral reflectors as well as multiple dichroic filters. U.S. Pat. No. 3,963,328 discloses a wide angle reflective optical system to provide corrections for spherical aberration, coma, and astigmatism by use of reflective elements only.